Allow plugins ability to augment the html displays?

[jonathan]

I checked in some further improvements, and created an example plugin that adjusts the schedule background to show local sunrise and sunset times. Validation that the mechanism is practical. I haven't tested the installation and the correctness of the manifest. (Plugin management features don't work on my Windows dev environment.)

The plugin code is here: https://github.com/jonathan-marsh/sip_pl...al_display